Method of detinning



Filed Feb. 22, 1945 7/ENTOR. www? Wm M gp. r r A Patented Oct. i6, i945 v METHOD F DETINNING v James P. A. McCoy, Milwaukee, Wis., assigner of one-half toMilwaukeeToolrDie Company.

Milwaukee, Wis.. a corporation of Wisconsin Application February 22, 1943, Serial No. 416,772

(Cl. 'I5-64) 1 Claim.

My invention relates to improvements in the art of separating adhering materials having different melting points, and relates more specifically to an improved method of removing tin or similar coatings from ferrous sheets or the like.

The primary object of my invention is to .provide an improved process of removing one adhering material from another in a simple and expeditious manner.

Many diierent methods of removing tin coatings and solder deposits fromferrous metals and especially sheet metal such as tin cans, by both chemical and mechanical treatment, have heretofore been proposed, and some of these prior methods have in fact been used commercially with varying degrees of success, but most of them have long been abandoned as being commercially impracticable. Because of the relatively low percentage of tin as compared to ferrous metal embodied in tin cans, it is necessary to treat large volumes of these receptacles in order to make the recovery of the tin a paying proposition; and while numerous attempts have heretofore been made to melt the tin coating oil of their bearing sheets and to thereafter drain the molten tin away, the high temperatures required to place the thin coatings into suillciently uent condition to permit such drainage also results in excessive oxidation and consequent negligible pure tin recovery. In order to avoid such excessive oxidation, it was heretofore necessary to heat the tin coated objects in specially confined zones quite devoid of Oxygen, but this procedure necessitated batch treatment of the cans and resulted in other complications and slowing up of the process, so that onlylimited volume of commodity could be handled. vThe chemical methods of salvaging tin are relatively impractical during normal times due to the cost of the chemicals necessarily used, which in some instances exceeds the value of the recovered products, and most of the prior recovery processes have therefore been `oirlectionable and impracticable both from the standpoint of dimculties encountered in exploitation and the excessive cost thereof.

It is therefore a more specic object of my present invention to provide a new and useful method of removing one metal from another having a different melting point, in a speedy and efcient manner, and without the use of chemicals.

Another speeic object of the present invention is to provide an improved process of continuously and effectively salvaging tin coatings, solder deposits, or the like, as well as clean ferrous sheet metal, all relatively free from oxides and in commercially practical volume and at minimum cost.

Still another specific object of this invention is to provide an extremely simple and eicient mode of' detinning coated sheet metal so as to recover substantially all of the pure tin in granular form, while also salvaging the sheet metal for further use. Y

An additional specic object of my invention is to provide an improved mechanical method of removing tin or the like from ferrous sheet metal, with the aid ofv relatively inexpensive removing agent which may be readily salvaged for reuse.

A further object of the invention is to provide a simple and effective process of heating metals in a reducing atmosphere while avoiding undesirable and excessive oxidation thereof, even when heated considerably beyond the oxidation temperatures of the metals, and without utilizing coniined chambers and other instrumentalities such as are ordinarily resorted to for such purposes.

These and other specific objects of the invention will be apparent from the following detailed description.

A clear conception of the several steps of my improved process. and of the procedure in carrying on lthe improved method commercially, may be had by referring to the drawing accompanying and forming a part of this specification wherein like reference characters designate the same or similar parts.

The single ligure of the drawing represents a typical now sheet showing diagrammatically the several steps and simple equipment involved.

While my invention has been shown and described herein as being especially applicable for the purpose of reclaiming tin and ferrous metal from tin cans or the like, it is not my desire or intent to unnecessarily restrict the scope or utility of the improved process by virtue of such specific embodiment, since the improvement may be adapted for advantageous treatment, and separation of other adhering materials having different melting points. The terms tinf solder," and ferrous" metal should therefore be given the broadest possible interpretation consistent with the prior art.

In accordance with my improved method as applied for the recovery of tin, tin coated or plated objects such as tin cans are preferably reduced in size and heated while constantly advancing through and while being continuously shaken and agitated within a heating none to a vtemperature sumcient to plasticize the tin, but

without .materially aectlng the solid condition of the ferrous sheet metal, in' the presence of a wiping agent lsuch as silica gel or ilour, bentonite, carbon black. silicon carbide, or other similar material, reduced to ilnely divided state or pulverulent condition. The pulverulent agent may be mixed with other carbonaceous material such as sawdust which will carbonize at the melting temperature of the tin, and will thus produce a reducing atmosphere by converting the oxygen in the vicinity of the metal to carbon dioxide; while at the same time enhancing the wiping action by increasing the volume of the wiping mixture and thereby insuring contact with all portions of the plasticized metal coatings. The wiping mixture used should preferablyhave con.. siderable bulk in order to properly cover and conceal the metal, and should be of such character that it will readily remove and mix with the plastic tin; and this mixture should also be of such nature that it may be readily subsequently separated from the -tin particles by notation or other simple mechanical means. After the tin coated objects have been thus heated and agitated in a reducing atmosphere and sufficiently to cause the wiping agent to intimately contact and remove the plastic tinfrom the ferrous metal, the tin laden wiping mixture consisting of pulverulent and `unburned carbonaceous agents, may be readily separated from the solid ferrous metal objects by screening and continued agitation, and the ferrous metal will be in clean and relatively unoxi'dized condition while the tin laden wiping and protective agents may be deposited in a liquid bath or subjected to other separation with the aid of a fluid. This deposition of the mixed wiping agents.` and tin will cause the, tin particles to promptly settle by gravity to the bottom of the liquid bath, while the wiping and unburned carbonaceous agents will iioat together with other removed impurities.

same. In the treating chamber I, the tin coat ing and solder deposits are plasticized by the heating of the drum, and the wiping mixture quickly removes these plastic metals from the ferrous metal due to the continuous contact and agitation, while the sawdust carbonizes and removes excess oxygen. The bulk mixture is thereafter passed through a wiping agent remover i which may be provided with a. separating screen or the like for the tin laden wiping mixture, and with discharge means for the ferrous metal pieces. These ferrous metal pieces may be delivered to a baling press. t where the steel or iron scrap is compressed into compact bales. while the separated wiping mixture may either be returned to the treating chamber 4, or sent to a wiping agent and tin separator 1. It is preferable to recirculate the wiping mixture through The subsequent removal of, the settled granular tin particles and the reclamation of the iiot-y ing wiping agents for subsequent use, are simple matters, and the improved process may thus be carried on continuously and rapidly as it takes i be employed in the commercial exploitation of my improved process, and the use of different types of pulverulent wiping agents may be resorted to, the continuous exploitation oi' the improved process can be facilitated by utilizing relatively simple and well known equipment such as shown diagrammatically in the accompanying typical flow sheet. When tin cans are being handled, it is preferable to first pass the cans through a cutting machine 2 for reducing them to relatively small pieces all parts of which will be exposed to the wiping mixture. 'I'he disintegrated cans may then be mixed withpulverulent wiping agent and sawdust or the like in a mixer 3, from whence the combined mixture may be fed into a-treating chamber l.` This chamber l may be in the form of an externally heated tumbling barrel or drum rotatable about an approximately horizontal or slightly inclined axis, and adapted to constantly advance the bulk mixture along its axis while thoroughly agitating Vthe 75 the treating chamber 4 several times so that the same will become saturated or thoroughly laden with granular tin, and the separator l may consist of a bath of water or other liquid in which the granular tin and solder particles will quickly settled by gravity, while the wipingvagent may be removed by flotation and reclaimed for further use after 'being dried.

From the foregoing detailed description it will be apparent that my present invention provides an improved method of detinning which is simple and eiiicient, and which canbe carried on commercially in a continuous manner and at extremely moderate cost. Actual exploitation of the improved process has proven that the tin coatings and other metals having relatively low melting points may be quickly and most effectively removed from ferrous metals such as ferrous sheet materials in but a few minutes with the aid 0f simple and relatively well known inexpensive equipment, and the wiping agent can be used over and over for a long period of time. By virtue of the fact that the granular tin may be separated from the wiping agent by settling, and that the agent can be recovered by flotation, no chemical treatment is required, and in order to prevent excessive oxidation of the metals when the tin is heated to the plastic stage, it is only necessary to add suihcient sawdust or other material which will maintain the desired reducing atmosphere Y within the heating and wiping zone. Besides the wiping agents specifically mentioned herein, various clays and other pulverulent materials may be utilized, but in any event the wiping agent should be fluffy and loose and reduced to approximately. meshv or ner in order that it will effectively spread and come into intimate wiping contact with the plastic tin and solder. The selection of a ,wiping agent should be determined by the desired condition of the finally recovered metal, since the neness of the granular tin may be varied considerably by utilizing different wiping agents. It has also been found that excessive heating such as might cause the sawdust to burn violently,is not necessary in order to plasticize the tin and solder, and that the u se of the sawdust or other material which will carbonize and constantly remove oxygen, eifectively prevents oxidation of the ferrous metal and assures production of scrap which is relatively free from oxide. In case the melting point of the material being treated, is considerably above the carbonizing temperature of sawdust, then it is necessaryto utilize some other less combustible carbonaceous material in order to produce the desired reducing atmosphere, but sawdust has been found satisfactory for the treatment of tin cans and the like. The reclaimed 1y agitating and heating'the coated metal to a temperature sumcient to plasticize only the tin coating while subjecting the softened coating to the rubbing action of silica gel in quantities only sumcient to remove the tin and while also maintaining the removed tin constantly concealed from atmosphere within an abundance of wood sawdust mixed with the silica gel in quantities sufiicient to prevent oxidation o! the tin and ferrous 10 metal. and thereafter removing the tin in gram ulated condition from the mixture ot silica gel and saw-dust. n

JAMES P. A'. McCOY. 

